A body’s state of decomposition, can indicate the time of death. This is easy to estimate within 12 to 24 hours of death, but later than that, investigators must use indirect evidence&colon; such as chemical signatures or insect colonisers like blowfly maggots and flesh-eating beetles. The accuracy of dating with maggots has recently been called into question, however. In the absence of these clues – in burnt remains for example – radioisotopes can reveal time of death.

Vital clues

Injuries can hint at crimes in the living, the dead – and even the long dead; such as 5000-year-old Ötzi the ice man, discovered in the Alps in 1991. Analysis of stab wounds, for example, can tell whether the blow was meant to be fatal, and if the wound is the product of murder or suicide.

Bite marks are present in 8 out of 10 sexual assaults and many homicides in the US. Forensic odontologists attempt to match these to suspects’ teeth. Bite marks left on chewed objects, such as food or pencils, can even link people to crimes. However, critics argue that there are no universally consistent methods for comparing bite marks. Now, a new animated method to create 3D reconstructions of marks is helping to make this less subjective and more accurate.

Blood is often found at crime scenes, and measuring splatter from gunshot wounds can give vital clues about what happened. Forensic scientists can also check for semen stains – a trick Japanese wives can use to reveal cheating husbands.

Fingering suspects

Matching fingerprints is one of the best-known methods of linking suspects to crimes. Devised in the 1800s, the technique made its UK court debut in 1902. Fingerprints consist of oil, sweat and skin cells, which leave behind an impression of the unique patterns of whorls and ridges each finger possesses. Even “invisible” or smudged sweaty prints can prove useful, and chemical composition of fingerprints can reveal suspects habits too, such as whether they are a smoker, or which aftershave they wear.

Fingerprints are commonly left at crime scenes. Explosive devices are even dusted for them by bomb disposal robots, before they are destroyed. But the method of matching fingerprints has scarcely been subjected to the objective scrutiny needed to determine how often bogus matches are likely to happen – though the first test of this was published in 2005. This is partly because the technique is so long-established, but some experts argue that fingerprints could be mismatched a shocking 20% of the time.

Forensic scientists also search for firearms resides on suspects that suggest they have fired a gun – however, this technique may have led to false convictions in the past. Explosives residues can be detected in the same way, but were used to falsely imprison the UK’s “Birmingham six“, bringing that aspect of forensic chemistry into question.

DNA revolution

DNA fingerprinting, or profiling, is now superseding traditional fingerprint matching as a more rigorous method. First developed in 1985 to diagnose genetic illness, it is now commonly used in criminal investigations. The first UK arrest following a DNA match came in 1995; since then, the England and Wales National DNA Database – the largest in the world – has matched more than 600,000 people to crimes.

DNA – from as little as a single cell – is extracted from blood, semen, tissue, hair or saliva. It is analysed in the lab to derive a characteristic fingerprint, or pattern of repetition, for certain strips of non-coding repetitive DNA. This is then compared to DNA profiles held in a database, and when two samples match, they likely came from the same person or sometimes a close relative. One day these databases may hold profiles for the entire population, raising privacy issues and stirring controversy.

Until recently, samples containing the DNA from several people, have defied analysis – a problem because contamination is common. But now, a new supersensitive method of computer analysis can tease some of these apart. This was introduced to some police forces in the UK in 2006 and may help solve thousands of unsolved crimes.